"Lip smacking, a common primate facial gesture used in friendly interactions, involves rapid opening and closing of mouth parts in a speech-like fashion. However, geladas are unique because they simultaneously vocalize while lip smacking to produce a sound that has been called a "wobble."

The gelada wobbles have a rhythm that closely matches the pacing of syllables spoken by humans.

The wobble is used primarily by males and always in a friendly context.

Some other primates such as apes and monkeys produce complex sounds, but they don't have the speech-like rhythm that geladas have."

The BBC says the wobble sound is like a cross between a yodel and a baby's gurgle. It reminds me of someone saying "Yeah, yeah, yeah" real fast in response to something you said. It's the gelada version of "I know, I know, I know."

"What's incredible about Thore's study is that he showed that under certain circumstances [geladas] combine rhythmic facial expression and vocal chord movement," said Asif Ghazanfar, an assistant professor at Princeton University who also studies the biology of primate communication.

The really interesting question, said Ghazanfar, is why is it only seen in geladas? "What is the change in circuitry that allows them to coordinate facial expressions and vocal expressions? What is it about geladas that allows them to do that?"

Vocal prowess isn't the geladas' only unique feature. They also travel in large groups, several hundred strong, "an order of magnitude larger than even the largest baboon groups," according to Bergman. Wobbling and other verbal exchanges may be employed as a method of bonding "in a similar way to how humans might use small talk." And both male and female possess a red patch of skin on their chests, "something that no other primate has," lending them the name "bleeding hearts."